(a) Field of the Invention
The present invention concerns current collectors for safe electrochemical generators, their processes of preparation and generators obtained utilizing these current collectors. More specifically, the present invention concerns current collectors, with lateral collection, consisting of a conductive coating supported on an insulating plastic, characterized in that the supported metallic coating is divided into a plurality of conductive areas which are insulated from adjacent areas, but are connected there between by means of a common conductive margin connected by means of conductive bridges capable of limiting or interrupting the total current received by a given area, originating from the whole of the generator, when the latter area is in a short circuit situation. The invention also comprises a process for producing such collectors and generators produced with these collectors.
(b) Description of Prior Art
Primary or rechargeable generators with high densities of energy are capable of reaching, in case of accidental rapid discharge, conditions of temperature and, possibly, pressure such as the battery may in certain extreme cases become dangerous and may even explode. Even if these extreme conditions are not always achieved, permanent damages may result to the generator.
This type of problem has increased with the introduction of lithium generators, which generally utilize liquid electrolytes and sometimes soluble cathodes. These generators contain sufficient energy so that certain designs, under conditions of major short-circuits, undergo an internal temperature increase of a few hundreds of degrees. Boiling of the electrolyte and especially, chemical reaction between lithium and the electrolyte or the cathode, may then become sudden and explosive at high temperature, particularly when the temperature exceeds the melting point of lithium, at about 180.degree. C. This situation is particularly serious in the case of rechargeable lithium generators where the contact surface and the reactivity of lithium with the liquid electrolyte increase substantially with the number of cycles of recharge.
Other solutions have been proposed to control these problems of safety: utilization of fuses in order to interrupt or limit the current in the case of external short-circuits, replacement of fusible metallic lithium with alloys or compounds of insertion, such as LiC.sub.6, which are not very fusible.
The protection of these generators against internal short-circuits is more difficult to obtain. An interesting solution has been proposed for lithium generators, in European Patent No. 0 246 866. This protection is based on the capacity of the porous plastic membrane containing the electrolyte of the separator, to melt starting from a temperature selected from below a temperature considered critical.
The present invention mainly concerns lithium generators, in solid state, utilizing electrodes, electrolyte and collectors in the form of thin films, which are produced, preferably by means of continuous processes and wherein the total thickness varies more often between 200 and 5 microns. Polymer electrolytes are already considered per se as being safer because of the solid state and their capacity to limit rapid exchanges of materials between the electrolyte and the electrodes; on the other hand, the latter are particularly suitable for use in collectors of the present invention.
The invention aims more particularly at current collectors for generators, in the form of thin films which require the development of batteries with large surfaces. These generators will more often be assembled in the form of windings or stackings and should be laterally collected in view of the length of the films to be used for storing appreciable amounts of power energy. These conditions prevent in practice the collection of currents from electrodes through the extremity of the electrode collector.
The collectors of the present invention therefore aim at improving the safety character of these types of lithium generators. Their aim is to limit the heat generated in the latter, in case of internal short-circuit, by temporarily or definitely limiting the maximum short-circuit currents which originate from the whole of the generator. This limitation is obtained by dividing the surface of at least one of the collectors of the generator into a plurality of distinct areas which are connected to a common conductive area by means of conductive bridges selected so as to permit on the one hand, the passage of currents corresponding to the normal operation of the generator and to limit or interrupt the current received by an area in case of short-circuit inside this area. According to the embodiments of the invention, the limitation of the current will enable to prevent a dangerous situation in case of accidental short-circuit, or still, it will enable to definitely isolate the defective area if the bridges are definitely melted, in which case, the generator will be able to continue to operate with a reduced capacity within the ratio of the surface of an area to the total surface of the areas of the generator.
To facilitate the production of these safe collectors, the invention utilizes collectors which are supported on plastic film. The use-of these support films is easier in all-solid systems, such as generators with polymer electrolytes. Moreover, the plastic support facilitates the handling of cut out collectors, which are otherwise difficult to use, and may serve permanently to isolate the various components of the generator, for example with wound batteries, where the support acts as insulating material between superposed batteries. Similarly, the support film may facilitate the handling of collector-electrode units and be used as a support for assembling the other components and the entire generator.
An additional aim of the invention is to provide a preferred process for producing this type of safe collector which is particularly adapted to generators with thin films, in that it enables to adjust the surface conductivity of the collectors while reducing at a maximum the excess of weight or volume of the collectors. In addition to this possibility of optimizing the density of energy of the generator, this process facilitates the use of polymer electrolyte generators, according to processes inherent to films of plastic material, for example, with respect to the manufacture, control of quality and temporary storage of large mother-rollers of complexes of multi-layer films assembled in the form of batteries.